All digital communication channels respond to a digital bit stream representing the intelligence to be transmitted, and convert that bit stream into an analog waveform which waveform depends both on the modulating intelligence and the particular modulating technique employed, i.e., FSK, PSK, and the variations on each of these techniques, i.e., two-phase PSK, four-phase PSK, etc. This modulated signal may itself be employed as a modulating signal for a carrier so that modulated waveform can be up-converted into the desired communication channel. At the demodulator, the inverse process is carried out and thus up-converted signals are down-converted to provide an analog waveform, i.e., baseband, which is operated on to recover a data signal (representative of the transmitted data) and a clocking signal (i.e., symbol clock). The combination of analog waveform and clocking signal is then employed to recover the intelligence in the waveform itself. Throughout this process, i.e., beginning at the modulator, continuing in the communication channel coupling modulator and demodulator, and in the demodulator also, noise corrupts the signal desired to be transmitted. This is manifested in the analog waveform of the demodulator, by deviations in magnitude from a theoretical magnitude which can lead to errors in the process of converting the demoudlated signal to represent the transmitted data. Since the intelligence in the analog waveform is recovered by sampling the waveform at the symbol clock, the magnitude of the analog waveform at those points in time is the basis for the remaining portion of the demodulation process. Furthermore, other conventional circuits are employed for AGC and DC level correction so that peak-to-peak and DC level of the analog waveform are adjusted to nominal levels. Typically, a demodulator will recover the intelligence from the analog waveform by noting the polarity of the waveform at the times of the symbol clock.
The communication art has long been aware of the value of determining quantities such as signal-to-noise ratio, carrier-to-noise ratio, or energy per bit to noise ratio of a communication channel. Any of these quantities provide an indication of the quality of the channel or the capability of the channel to communicate information from a transmitter to a receiver.
Until very recently, however, the only known techniques for measuring these parameters made it necessary to terminate communication of information, and instead, to transmit a known signal along the channel for measurement purposes. This is undesirable for at least two reasons; firstly, when a measurement is being made, the communication channel is incapacitated; and secondly, a measurement only determines the quality of a channel during the time of the measurement interval, and since the quality of a communication channel is a function of time, the measurement user is necessarily forced to interpolate or estimate from the measurement what the quality of the communication channel will be when it is actually transmitting intelligence.
More recently, Keelty and Feher, in "On-Line Pseudo-Error Monitors for Digital Transmission Systems" in IEEE Transactions on Communications, Vol. COM-26, No. 8 (August 1978), page 1275 et seq., suggest a technique of pseudo-error detection. Pseudo-error detection involves a secondary decision device, connected in parallel with the main data path. In the implementation suggested by Keelty et al, the secondary path is intentionally degraded and thus, the signal sequence on the secondary path has an error rate much greater than the unknown error rate of the channel being monitored. If a user knows the relationship between the degraded secondary path and the main channel, a relationship can be drawn between the error rate on the degraded secondary path and the desired figure for the channel being monitored.
The accuracy of the result of this process depends, in part, on the accuracy of the knowledge of the relationship between the degraded secondary channel and the channel being monitored. In addition to this source of uncertainty, the technique suggested by Keelty et al to implement the degraded secondary channel requires the addition of circuitry of significant complexity, and therefore, cost.
We disclose hereafter what we believe is a more effective technique, namely, to monitor signal magnitude deviations on a finer level which enables us to reduce monitor complexity and still maintain accuracy. In particular, the Keelty technique is to degrade the baseband signal to produce more frequent "errors," which are defined as a positive signal going negative, or vice versa. On the other hand, we eliminate the necessity of adding signal degradation by counting as an "error" analog signal variations over a much smaller range.
It is one object of the present invention to provide a simple but effective on-line channel quality monitor which does not require interruption of channel communications. It is another object of the present invention to provide such a channel monitor which is capable of monitoring E.sub.b /N.sub.0, in a simple but effective and accurate fashion. It is a further object of the present invention to provide for the foregoing apparatus with the addition of relatively simple and inexpensive components. Other and further objects of the invention will become apparent as the description proceeds.
The invention has applicability to any communication channel where an analog sample of the demodulated data is present (commonly known as an eye pattern). The digital channel which necessarily transmits only samples clearly satisfies this requirement and the parameter measured by the inventive circuit is E.sub.b /N.sub.0 (ratio of energy per bit to noise). In an analog channel, meeting the foregoing requirment, the parameter measured is S/N or C/N (signal to noise ratio or carrier to noise ratio).